Method of producing cold rolled steel sheets suitable for press forming



Jan. 6, 1970 METHOD OF PRODUCING COLD ROLLED STEEL SHEETS SUITABLE FOR PRESS FORMING Filed June 2, 1966 SHUN FUJIMOTO EI'AL 6 is lb [2 I11 SURFACE ROUGHNESS Kyla/M w Armme'v:

United States Patent 3,487,674 METHOD OF PRODUCING COLD ROLLED STEEL SHEETS SUITABLE FOR PRESS FORMING Shun Fujimoto, Aichi-ken, and Kazutami Yasunaga, Hyogo-ken, Japan, assignors to Fuji Iron & Steel Co., Ltd., Tokyo, and Toyota Motor Co. Ltd., Toyota-shi, Aichi-ken, Japan Filed June 2, 1966, Ser. No. 554,710 Claims priority, application Japan, June 6, 1965, 40/ 33,697 Int. Cl. B21!) 1/38; B21d 51/00; B23p 13/04 US. Cl. 72-366 4 Claims ABSTRACT OF THE DISCLOSURE A method of producing cold rolled steel sheets in which the surfaces of the steel sheet are formed with one surface having a coarser surface roughness than the other to a degree greater than the ordinary roughness variation in such sheets and the steel sheets are press formed in a die whereby the rougher surface is contacted by the die pressing surface.

SUMMARY OF THE INVENTION The disclosure relates to a method of producing cold rolled steel sheets suitable for press working, in which the difference in surface roughness between the front surface and the back surface of the sheet has been made greater than the ordinary roughness variation.

In general, hot rolled or cold rolled steel sheet is used for press forming depending upon the requirement of finished parts or applications, and ordinarily such sheet material is first formed into required shapes, assembled, and then painted or plated. Accordingly, for steel sheet to be used for press forming, its workability, and coatability the well as economics of the operation are matters of importance, and, particularly for the applications where the finished appearance is important, it is most desirous that the steel sheets to be used in such applications have good workability, forming and coatability and do have not surface scratches, unevenness and other defects which spoil the surface appearance.

Various properties of the steel sheet, such as good flange stretchability and deep drawability, are required according to the shape of finished products, and, in addition, good coatability and good surface luster are required in order to enhance the commercial value of finished products. It is widely known that cold rolled steel sheets which can meet these requirements as closely as possible are useful for press forming operations.

However, to date cold rolled steel sheets for press forming have had smooth surfaces on both sides or rough surfaces on both sides, but in either case the difference in surface roughness between the front and back surfaces is less than 1.5,uH max. which is within the ordinary range ofvariation, and thus the surface roughness of the both surfaces are nearly the same, which fails to meet completely the above-mentioned requisites of the cold rolled steel sheets for press forming operations. The reason for this is that the surface roughness required for obtaining the desired workability and formability in pressing operations is contrary to the surface roughness required for obtaining good surface luster after painting or plating.

1) Namely, in the first place, a rough surface on the sheet provides desirable conditions for press forming, since the lubricant stays in the depressions of the surface irregularities and forms a film, thus increasing the lubricating effect. From the point of coating with paint, the rough surface assures good adhesion of the coating, but

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an exceedingly rough surface will retain the surface roughness even after plating or bright coating and gives bad surface luster and, thus is unsuitable for affording a beautiful appearance, and uneconomical because a greater amount of coating material is needed. Meanwhile during the production of steel sheet having which has a considerable surface roughness there is no difficulty of slipping as is encountered in the smooth rolling, thus less surface damages due to the slipping results, and thus the rolling operation is easier to perform and desired rolled sizes and shapes are easier to obtain, while surface damage, if any, is inconspicuous. Further, during the annealing operation, there is almost no danger of sticking even when the annealing is done at a higher temperature than the transformation point of the steel to be annealed.

(2) Steel sheets having a smooth surface will give good surface luster and beautiful appearance after plating or painting, but they are inferior to a steel sheet having rough surfaces in respect of workability, formability, and surface damage.

From the facts set forth above in (l) and (2) it is apparent that rougher surfaces on steel sheet are preferable for obtaining better workability and formability, while smoother surfaces of steel sheet are preferable for obtaining better surface luster after plating or painting. Therefore, the conventional cold rolled sheet for press forming having almost the same surface roughness on both sides cannot provide the conditions which produce good workability and formability or at the same time good surface luster after plating and painting.

For the purpose of satisfying these contradictory conditions, the present invention has as its object to provide new cold rolled steel sheet suitable for press forming which provides improvements in sheet production, good workability, and formability as well as good surface luster after plating or painting, and such new steel sheet can be provided by making the roughness of one surface of steel sheet different from the other. For instance the surface forming the outer surface after press forming, must have a roughness not in excess of 6,uH max. (this includes so-called smooth surfaces). Similarly the roughness of the other surface, the inner surface after press working, must be not less than 4/LH max. Moreover, the difference in the roughness between the surfaces will be about 3,uH max. Further, the present invention has as its object to provide plated or coated products which may be obtained by painting or coating the smoother surface of sheet obtained by press forming whereby in the pressing operation the press punch contacts the rougher surface of the sheet.

The present invention has the further object of providing methods for making such new cold rolled sheet suitable for making press formed products from such new cold rolled sheet.

To explain the indication of the surface roughness as used herein, the maximum height of surface unevenness is measured and expressed in a and the surface roughness is expressed in term of ,uH max. according to Japanese Industrial Standard B0601. The maximum height of surface unevenness means the distance from the basic surface (in case of a flat finished surface, the plane which come in contact with the flat finished face at three points or more) to the deepest dale or depression opposite to the surface. (A large peak or dale which occurs not more than one time in the standard area of the basic surface shall be excluded.) In the case of differences in shape or size of the unevenness due to the direction, unless otherwise specified, the maximum height in the roughness curve when out in the direction of the largest unevenness is considered as the roughness for that surface. The surface roughness is indicated by the arithmetical average of the roughness measured in several portions, and the individually observed value may infrequently be of 50% increase above the mean value of roughness.

In the present invention the surface of steel sheet which is to be the outer surface when the sheet is pressed and assembled is, as mentioned before, defined as not more than 6 H max. (this includes so-called smooth surface). The reason for this limitation is that good surface luster after painting is required for applications such as auto trims and electric appliances and good surface luster after plating is required in case of auto bumpers and other parts which are plated after press working, and for these purposes, a natural smoother surface such as a glass-like surface is better, and the reflection measure of optical scattering must be 0.2 or more in order to maintain good surface luster after painting or plating. However, on the other hand, a somewhat rougher surface of steel sheet, as mentioned before, is required for obtaining improved bond between the coating material and the steel surface. Further, where the steel sheet has a relatively rougher surface the rolling operation is easier, desired rolled sizes and shapes are easier to obtain, and surface damages are almost negligible, there is almost no danger of sticking during the annealing operation, and the desired effects of skin-pass rolling are easily obtained. Thus, the roughness of the surface of steel sheet which is to be the outer or supported surface when the sheet is press formed and assembled is defined as not more than 6,LLH max. from the above contradictory requirements.

On the other hand, the reason for defining the roughness of the surface of steel sheet which is to be an interior surface when the sheet is press formed, that is the surface against which the pressing action is elfected, as not less than 4,uH max. is that, although a rougher surface will give better workability and formability due to its better lubricating effect, when the roughness is less than 4,uH max., particularly in case of cold rolled steel sheet for press forming to be used in auto and electric industries, the aforementioned advantages due to the surface roughness will be lost and workability and formability will be lowered.

The difference in the surface roughness of more than 3,uH max. means such relationships that when and if the roughness of the smoother surface is 6,uH max., the roughness of the rougher surface is more than 9,uH max., 12 14/.LH max., for example, or if the roughness of the rougher surface is 4,1LH max., the roughness of the smoother surface is less than IuH max., 0.50.8,uH max., for example. The reasons for defining the roughness difference as not less than 3/.LH max. are based on the facts that when we compare the cold rolled sheet according to the present invention with the conventional cold rolled sheet for press forming in which the roughness difference is less than 1.5,uH max. which is within the ordinary roughness variation, it is found that better workability and formability is obtained when the roughness difference is larger than the ordinary roughness variation and that when the roughness difference is more than about 3,uH max. such advantageous effects are remarkable.

It is possible to obtain similar workability and formability by roughening the both surfaces of the conventional steel sheet for press forming operations. However, it is apparent that the surface luster after plating and painting is inferior to that obtainable by the present steel sheet.

As clearly understood from the above description the present invention has as its main object to provide cold rolled steel sheet suitable for press forming which steel sheet has excellent workability and formability as well as excellent surface luster after plating and painting.

It is now made possible, in accordance with the present invention, that similar pressed parts, which have been hitherto made by using high-cost deep-drawing steel sheet, can be produced by low-cost drawing steel sheet. Further, in the prior art, good surface luster after plating or painting was obtained where both surfaces of the steel sheets used in the press forming operation had similar smooth finishes. With such steel sheet, very often surface damage occurred due to the foreign substances introduced between the press die and the steel sheet during the press forming and thus it was necessary to recondition the surface by hard working. Whereas, when the present steel sheet is used foreign substances will be entrapped in the cavities of the surface irregularities and thus surface damage due to such substances is inconspicuous or no such damage occurs, which substantially reduces the hand work labor for reconditioning.

Now, the method for producing the present steel sheet will be described hereinafter.

It is common practice for producing cold rolled steel sheet that steel is refined using an ordinary steel refining furnace such as a converter and an open hearth furnace, is tapped and, molded into ingots and further, is worked into slabs by breaking-down rolling operations. Then the slabs are hot rolled into hot rolled coils which are then subjected to acid pickling to remove oxide surface scale, cold rolling, annealing and finally skin-pass rolling to obtain finished products.

According to the method of the present invention the cold rolling and the skin-pass rolling in the above sequence steps are done maintaining the difference in roll surface roughness between the upper working roll and the lower working roll at not less than 4,1LH max.

It is noted that during the cold rolling, the transcription efficiency by the working rolls is not complete due to the influence by the lubricant, the forward slip etc. and (even when the surface roughness of the working rolls is relatively rough) the extent of the surface roughness of the steel sheet tends to be less than that of the working rolls. Thus, in order to obtain a surface roughness difference not less than 3;1.H max. it is necessary to maintain the surface roughness difference between the upper and lower working rolls at not less than 4uH max.

In so far as the surface roughness difference between the upper and lower working rolls is maintained at not less than 4,uH max., even when one of the working rolls has-a smooth or nearly smooth roll surface, it is possible to obtain steel sheets having a smooth or nearly smooth surface on one side which maintains the advantageous features that desirable rolled sizes and shapes are easy to obtain, there is almost no danger of sticking during the annealing, and during the skin-pass rolling sheet form connection and prefixed amount of skin-pass working are easily effected.

For the purpose of providing a desired roughness on the roll surface, the roll surface is ground using a #120 grinder within a roughness range of about 1.5-0.8,MH max. and then subjected to the action of grit blasting to obtain the desired surface roughness. The particle size of grit blast may be selected according to a desired surface roughness. Table I illustrates one example of the relationship between the particle size of grit blast and the surface roughness of the roll as well as the surface roughness of the steel sheet.

TAB LE I Surface roughness of Surface roughness of Grit Particle size (No.) roll (14H max.) steel sheet H max.)

TAB LE II Surface roughness of roll (,nH max.)

Roughness of Grinder Further, for similar reasons, low-cost drawing steel sheet of rimmed type can also be used in place of deep drawing steel sheet of killed-type, and low-cost lubrieating oil of a lower quality can be safel used.

In addition, -a good surface luster can be obtained 1.0 0.7-1.0 85 6 2:3; with a small amount of paint, because the surface roughness of the outer surfaces as press worked and assembled It is noted that the above tables are only for the pub is suited for this surface. Also an excellent surface luster pose of illustrating examples and various degrees of can be obtained when platmg is employed surface roughness which can be obtained by changing 1O EXAMPLE II factors such as the combination of the working rolls of A hot rolled Steel coil was acid pickled to remove the cold oll'n a (1 th 0 k rolls of the tem er rollin the iollin re ducti or l rhte ind use of lubric nt duroxlde Scale a cold rolled up to a reduction using g the p: rolling a tandem mlll, the lower and upper working rolls of whlch Now the Present invention shall be further described 2 ggi g g g t f i ig gfi i z gygfi through the following examples. The attached drawing had been subjected blasting g G25 pgrit to ig i g igf 2 23 git i fii zg figf g of have a roughness of 25 H max., then annealed at 750 C. P and finally subjected to temper rolling by a skin-pass mill EXAMPLE I in which both the upper and lower rolls have a smooth Table III shows comparative data between the present roll surface, whereby a cold rolled steel sheethaving cold rolled steel sheet having different surface roughness surface roughness of on the coarsg slde and on both of its sides and a conventional steel sheet having Surface 0f on the other or Smooth almost the ame urface roughness on both of its sides Slde was obtained. T1118 St1 sheet showed much ltiSS in respect to their Erichsen value, Rockwell hardness and Su ce damages due to the sllpplng and the surface conical cup-test value. damages were much more inconspicuous as compared TABLE III Roughness of smooth surface (14H max.) 1. 4 1. 4 2. 5 5 4. 8-6 4. H Roughness of coarse surface (#11 max.) 14 6 5 16 18*22 (9 Direction of measurement B A B A 13 A B A B Erichsen value HRF 11.2 10. 7 11.0 10.7 11.0 10. 9 11. 3 10.9 11.6 10.9 10.7 Rockwell hardness 84.5 79.1 80.5 79.1 82.5 81.2 85.0 82.3 85.2 82.2 82.3 Conical cup value 37.93 30.50 37.63 36.56 37.25 36.80 38.10 37. 25 38.19 37. 20 37. 20

Remark: A of Direction of Measurement indicates that the press punch is put on the coarse surface and B indicates that the punch is put on the smooth surface.

1 On both sides.

Referring to Table III, the Erichsen value has relationship with the flange stretchability of the sheet during press forming and apparently higher values are shown where the punch is put against the coarser surface.

The conical cup value is a characteristic value which has relationship with the flange shrinking ability and remarkably higher values are shown where the punch is put on the coarser surface. As for the Rockwell hardness, higher values are obtained as the surface roughness increases.

Generally as the hardness increases the workability and formability decreases. However, the higher Rockwell hardness of the coarser surface of the present steel sheet is due to the higher Rockwell hardness of its surface portion. Its inner portion has a similar hardness value as does the ordinary steel sheet having relatively smoother surfaces. Due to this higher Rockwell hardness of thercoarser surface, higher lubricant effects can be obtained during press forming by putting the punch against the coarser surface, and uniform elongation can be obtained while harmful partial elongation is reduced. This means that a uniform distribution of strain is obtained after the press forming and thus local cracks are prevented.

As understood from the above, the present steel sheet is much more preferred than the conventional steel sheet in 'respect of workability and formability when press worked with the punch against the coarser surface, and this is particularly remarkable when the surface roughness difference between the both sides is more than 3 .1H

max.

As the steel sheet in accordance with the invention shows better workability and formability during the press forming than conventional steel sheet, the number of applications of lubricating oil is lowered and thus the consumption of lubricating oil is reduced which results in a reduction in the amount of labor expended.

with those which were cold rolled between the upper and lower rolls which had similar smooth roll surface, and its rolled sizes and shapes were precise and no sticking was observed during the annealing operation.

EXAMP-LE III A hot rolled steel coil was acid pickled to remove oxide surf-ace scale and cold rolled using a tandem mill the upper working roll of which had been ground with a grinder to have a surface roughness of about l H max. and further was subjected to grit blasting with G18 grit to provide a surface roughness of 35 H max., and the lower working roll of which had been ground with a #120 grinder and further was subjected to grit blasting with G40 grit to provide a surface roughness of 15/.LH max., and the cold rolled sheet thus formed was annealed at 800 C. and then subjected to temper rolling by a mill the upper roll of which had a surface roughness of l5,uH max, whereby a cold rolled steel sheet was produced having a surface roughness of 18 1H max. on one side and a surface roughness of 4.5}LH max. on the other side.

The steel sheet thus obtained had only a small amount of a inconspicuous surface damage due to the slipping during the cold rolling and the temper roller, and its rolled size and shapes were precise. Moreover, no sticking was observed during the annealing at considerably high temperatures.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodies otherwise without departing from such principles.

What is claimed is:

1. A method of processing cold rolled steel sheets comprising the steps of forming the steel sheets with a difference in surface roughness on the opposite surfaces thereof so that the difference in surface roughness is greater than the normal range of about 1.5,uH max, in surface roughness between the opposite sides of the cold rolled steel sheets and pressing the steel sheet in a die wherein the smoother surface is contacted by the die support surface and the rougher surface is contacted by the die pressing surface.

2. A method as set forth in claim 1, wherein the steel sheets have a first surface and a second surface which is relatively coarser than the first surface and wherein the first surface has a surface roughness not exceeding 6,u.H max. and the second surface has a surface roughness at least 3,4LH max. greater than the surface roughness of the first surface and which has a surface roughness of not less than 4,4H max.

3. A method as set forth in claim 1, comprising applying a coating to the smoother surface of the steel sheet after the pressing step.

' having a surface roughness difference therebetween of not less than 4/AH max.

References Cited UNITED STATES PATENTS 2,315,657 4/1943 Ritz 295Z7 2,869,265 1/1959 Klein 2952 7 3,145,468 8/1964 Johnson 29528 X JOHN F.' CAMPBELL, Primary Examiner D. C. REILEY, Assistant Examiner US. Cl. XR. 29-528; 113-116 

